I have (2) 40's projects going(not finished)
(1) 46 COE(cab over) on a 93 Dakota chassis with the 5.2efi and 518 trans
and a 47 dodge panel truck sitting on another 93 dakota chassis.
there is a great area on these frames to shorten them, on my coe I shortened it like 2 feet.
and Gene is right you will need to move the engine and trans back around 6"

also remember you will need to get a new wiring setup or have to use one and ECU from the dakota truck to make the efi and trans work.
I haven't got that far yet.
there is a guy here on moparts that will redo your stock harness for you for a fair price.